Drug addiction is a serious and prolific mental illness involving persistent relapse despite sincere efforts to abstain. This research studies the neurobiological mechanisms that increase the propensity for relapse after prolonged abstinence to determine novel targets for potential therapeutic intervention to promote abstinence. Previous studies suggest that mu opiate receptor stimulation in the nucleus accumbens (NAc) does not play a role in cocaine-seeking behavior using rodent models of cocaine relapse. However, we found that NAc infusions of the endogenous opiate beta-endorphinwill trigger cocaine seeking primarily via activation of mu opiate receptors (MOR1) in the NAc. Importantly, we found that MOR1 expression in the NAc progressively increases from early to late cocaine withdrawal, coinciding with time-dependent increases (incubation) in cocaine-seeking behavior. Moreover, our preliminary data suggest that increases in MOR1 expression in the NAc are paralleled by increased expression of the beta-endorphinprecursor proopiomelanocortin (POMC) in the arcuate nucleus of the hypothalamus (Arc). The goal of the proposed research is to study the functional contribution of neuroadaptations in these endogenous opiate systems to the increased propensity for relapse in long-term cocaine withdrawal. Studies in Aim I will determine the ability of MOR1 stimulation in the NAc to trigger relapse behavior after early and late withdrawal from chronic cocaine self-administration. Intra-NAc infusions of beta-endorphinand the MOR1 agonist DAMGO will be tested for their ability to increase relapse behavior in both extinction and reinstatement phases of experimentation. Similar intra-NAc infusions of DAMGO will be used to assess MOR1 signaling in vivo as a biochemical parallel to behavioral experiments. Studies in Aim II will use in situ hybridization to measure POMC expression in the Arc, and In Gel Western Blot of beta-endorphinlevels in the Arc and NAc, after both early and late withdrawal times. The contribution of increased POMC expression and beta-endorphin synthesis to the propensity for relapse will be studied by neutralizing endogenous beta-endorphinrelease in the NAc with anti-beta-endorphin and the MOR1 antagonist CTAP in extinction/reinstatement tests. Stressful situations activate POMC neurons in the Arc, and trigger cocaine-seeking behavior, but the role of POMC neurons in relapse behavior has not been studied. Studies in Aim III will investigate the effects of selective activation of POMC Arc neurons on cocaine seeking in early and late withdrawal using an optogenetic approach combining inducible viral vector-mediated channel-rhodopsin-2 expression in mice expressing Cre recombinase under POMC promoter control. The role of beta-endorphinrelease in the NAc in relapse induced by activation of POMC neurons will be assessed with intra-NAc infusions of anti-beta-endorphin. Together, these studies will determine the role of persistent neuroadaptations in endogenous mu opiate receptor systems in the increased propensity for cocaine relapse in prolonged abstinence.